This invention relates to a field effect transistor having a gate oxynitride film containing heavy hydrogen atoms, and a manufacturing method thereof.
In an element such as an electrically writable, erasable, and programmable read only memory (EEPROM), an electric field of 10 MV/cm or higher is applied to a gate oxide film in reading/erasing the information stored in the memory, and the gate oxide film is used as a tunnel oxide film. When the gate oxide film is applied with so high electric field, electrons energized by the high electric field pass through the gate oxide film. Accordingly, the gate insulating film is required to have high voltage withstand capability. If the gate insulating film is not formed to withstand such a high electric field, the impurity such as boron in the gate electrode is introduced into the gate insulating film to reach the substrate surface though the gate insulating film. The impurity concentration of the substrate surface is deviated from the desired level thereby, and thus the normal transistor characteristics may not be obtained.
Conventionally, the manufacturing condition of the gate oxide film has been determined empirically: various oxide films are formed by varying parameters such as a forming temperature and the concentration of the oxygen contained in the atmosphere; the electric characteristics of the formed films are evaluated thereafter; and then the oxide film satisfying the desired specification is selected from the films. As the types of products are varied and the speed of the alternation of product generations increases, however, the above-mentioned empirical determination method of the forming condition of the oxide film becomes not so effective, and is beginning to increase the manufacturing cost.
It has been reported that the post-annealing process (for 30 minutes at 900.degree. C.) performed in a D.sub.2 atmosphere containing heavy hydrogen (DV), i.e., "deuterium" is effective to suppress the generation of the interface state at the interface of the gate oxide film and the substrate (N. S. Saks and R. W. Rendell, IEEE Trans. Vol. NS-39, pp. 2220-2229, 1992). The method disclosed by the report, however, relates to the gate oxide film alone.